Segmented Power Diode Structure with Improved Reverse Recovery

ABSTRACT

A power diode comprises a plurality of diode cells (10). Each diode cell (10) comprises a first conductivity type first anode layer (40), a first conductivity type second anode layer (45) having a lower doping concentration than the first anode layer (40) and being separated from an anode electrode layer (20) by the first anode layer (40), a second conductivity type drift layer (50) forming a pn-junction with the second anode layer (45), a second conductivity type cathode layer (60) being in direct contact with the cathode electrode layer (60), and a cathode-side segmentation layer (67) being in direct contact with the cathode electrode layer (30). A material of the cathode-side segmentation layer (67) is a first conductivity type semiconductor, wherein an integrated doping content of the cathode-side, which is integrated along a direction perpendicular to the second main side (102), is below 2·1013 cm−2, or a material of the cathode-side segmentation layer (67) is an insulating material. A horizontal cross-section through each diode cell (10) along a horizontal plane (K1) comprises a first area where the horizontal plane (K1) intersects the second anode layer (45) and a second area where the plane (K1) intersects the drift layer (50).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/EP2020/059271, filed on Apr. 1, 2020, which claimspriority to European Patent Application No. 19166711.2, filed on Apr. 2,2019, which applications are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor structures and, inparticular embodiments, to a segmented power diode.

BACKGROUND

Snappy recovery of fast power diodes has been investigated for manyyears in view of the need for faster power semiconductor devices withlow switching losses during transient periods. Fast recovery diodes aretypically used in combination with integrated gate-commutated thyristors(IGCTs), insulated-gate bipolar transistors (IGBTs) and gate turn-offthyristors (GTOs) as freewheeling diodes, snubber diodes and clampdiodes.

The depletion of remaining stored charge during the recovery period of apower diode results in a current discontinuity (chop-off), manifested bya current slope, dI/dt. The dI/dt operates on the circuit's inductance,leading to a voltage overshoot (V_(L)=−L dI/dt) which may result in thedestruction of the device. FIG. 1 shows the time dependency of forwardcurrent IF and voltage across a power diode during the recovery period.It can be seen from FIG. 1 that at the end of the reverse recoveryperiod, the diode shows snappy turn off behavior (snappy recovery). Thisis mainly due to an insufficient thickness of the device. The snappyrecovery is more pronounced mostly at lower temperatures (25° C.) andlower currents (<10% of the nominal current). The snappy recovery can bereduced by increasing the thickness of the device. However, this in turncauses an increase in conduction and switching losses.

The integrated gate-commutated thyristor (IGCT) has been established asthe device of choice for high power applications such as medium voltagedrives, pumped hydro, railway interties and power quality applications.Today, IGCTs have been optimized for voltage source inverters (VSIs),current source inverters (CSIs) and event switching (solid-state circuitbreaker) applications and are available as asymmetric, symmetric(reverse blocking), and reverse conducting (RC) devices. For VSItopologies, the asymmetric IGCT has the highest power level for a givenwafer size while the reverse conducting (RC) IGCT provides compactness(reduction in size and weight) and improves reliability (reduced numberof components) by monolithic integration of a power diode on the samewafer as the gate commutated thyristor (GCT).

In an RC-IGCT, as both IGCT and diode are on the same wafer and use sameelectrical pressure contacts (one at topside and another one at bottomside of the wafer), the choice of the starting silicon material islimited to one thickness. The thickness is governed by the devicevoltage and cosmic ray resilience ratings, as well as the dynamicbehavior of both GCT and diode parts. The latter includes the diodesnap-off behavior described above. While increasing the thicknessimproves all three of the mentioned aspects, it also increases thelosses in both operational modes. In a typical optimization of anRC-IGCT, the snap-off behavior dictates the minimal wafer thickness,which is why the motivation for improving this aspect of diode reverserecovery is twofold for an RC-IGCT: both the GCT- and diode parts willbenefit from lower operating losses through a thickness decrease.

From EP 3 029 737 A1 it is known a back surface hole injection typediode. By more effectively securing the effect of hole injection fromthe back surface of a semiconductor substrate, the performance of asemiconductor device is improved. In the diode formed of a P-N junctionincluding an anode P-type layer formed in the main surface of asemiconductor substrate and a back surface N⁺-type layer formed in theback surface of the semiconductor substrate, a back surface P⁺-typelayer is formed in the back surface, and a surface P⁺-type layer isformed in the main surface right above the back surface P⁺-type layer tothereby promote the effect of hole injection from the back surface.

From JP H06-29558 A an electrostatic induction diode is described, whichhas a planar structure suited for obtaining high breakdown voltage bysetting the planar structure using an electrostatic induction effect inone or both of anode and cathode regions and also by setting a life-timedistribution in a high-resistance layer. A life-time in the vicinity ofanode and cathode sides is set long and a life-time distribution isgradually set short as the distance from these regions increases. Thelife-time of the position deep from the anode and cathode sides is setcomparatively short so that the reduction of residual carrier isaccelerated. Therefore, it is possible to realize high breakdown-voltagediode with little inverse-recovery charge quantity and shortinverse-recovery time in addition to the effect of electrostaticinduction short-circuit.

From DE 3 631 136 A1 it is known a snap-off free diode with a relativelythin silicon layer by implementing the Field Charge Extraction (FCE)concept, i.e. by introducing diode cathode shorts or p⁺ regions at thediode cathode side. These p⁺ regions inject holes during tail (end)phase of the reverse recovery period and support the current and henceimprove the softness of the device in diode mode of operation withoutmuch influence on the performance of the device in GCT mode of operationHowever, the Safe Operation Area (SOA) or maximum controllable turn-offcurrent (MCC) capability of the device in GCT mode can be low in anRC-IGCT with an integrated power diode having a FCE design as then-buffer peak doping concentration is usually lower (<1·10¹⁶ cm⁻³) inFCE designs compared with conventional diode design without diodecathode shorts or p⁺ regions at the diode cathode side. Also, theleakage currents can be higher in these structures with FCE designs asthe gain of a parasitic pnp transistor increases with low buffer doping.In addition, the FCE design is more sensitive to temperature variationof the device. The FCE effect is more pronounced, i.e. the carrierinjection from the p⁺ regions becomes higher at higher temperatureswhich results in increased reverse recovery losses. The strongtemperature dependence of the FCE design results in a big difference inthe technology curves at Tjmax and at room temperature as a result ofthe long tail current of the FCE parts.

From the publication “Dynamic Punch-Through Design of High-Voltage Diodefor Suppression of Waveform Oscillation and Switching Loss” by Tsukudaet al, Proceedings of the 21^(st) International Symposium on PowerSemiconductor Devices & IC's, pp. 128-131, 2009, it is known a powerdiode design with a silicon oxide region embedded in an n-type cathodelayer to cover a portion of the cathode electrode. It is described thatsuch power diode design can suppress oscillation of current and voltageduring reverse recovery.

In “Optimization of Diodes using SPEED concept and CIBH” byPfaffenlehner et al, Proceedings of the 23^(rd) International Symposiumon Power Semiconductor Devices & IC's, pp. 108-111, 2011, it isdescribed a free-wheeling diode with improved surge current ruggedness.In this free-wheeling diode according to the SPEED concept an anodeconsists of highly-doped p⁺ areas which are located inside a low-dopedp-emitter area. At low current densities, hole injection comes mainlyout of both p-doped areas but is mainly determined by the p⁺ areas.However, this diode using the SPEED concept may exhibit snappy reverserecovery when the device thickness is low.

SUMMARY

Embodiments of the present invention relate to a segmented power diodeas well as to a reverse conducting (RC) integrated gate-commutatedthyristor (IGCT) including such a power diode.

In view of the above disadvantages in the prior art, embodiments of theinvention can provide a power diode that exhibits a fast and softrecovery (i.e., no snappy recovery) while reducing the conduction andswitching losses in a wide temperature range.

In one embodiment, a power diode comprises an anode electrode layer, acathode electrode layer and a plurality of diode cells arranged betweenthe anode electrode layer and the cathode electrode layer. A directionfrom the anode electrode layer to the cathode electrode layer defines avertical direction (the vertical direction is the direction of ashortest line connecting the anode electrode layer and the cathodeelectrode layer). Each diode cell comprises a first conductivity typeanode layer which is in direct contact with the anode electrode layer, afirst conductivity type second anode layer having a lower dopingconcentration than the first anode layer and being separated from theanode electrode layer by the first anode layer, a second conductivitytype drift layer forming a pn-junction with the second anode layer, asecond conductivity type cathode layer being in direct contact with thecathode electrode layer, and a cathode-side segmentation layer being indirect contact with the cathode electrode layer. The second conductivitytype is different from the first conductivity type. Therein, the cathodelayer has a higher doping concentration than that of the drift layer.

A material of the cathode-side segmentation layer is a firstconductivity type semiconductor or an insulating material. In case thatthe cathode-side segmentation layer is a first conductivity typesemiconductor, an integrated doping content, which is integrated along adirection perpendicular to the second main side, is below 2·10¹³ cm⁻².The first anode layer extends in the vertical direction from the anodeelectrode layer to a first depth and the second anode layer extends inthe vertical direction from the first anode layer to a second depthlarger than the first depth. A horizontal cross-section through eachdiode cell along a horizontal plane perpendicular to the verticaldirection at a third depth comprises in each diode cell a first areawhere the horizontal plane intersects the second anode layer and asecond area where the horizontal plane intersects the drift layer,wherein the first depth is smaller than the third depth, and the thirddepth is smaller than the second depth. The drift layer may exemplarilyhave a constant doping concentration. Therein, a constant dopingconcentration means that the doping concentration is substantiallyhomogeneous throughout the drift layer, however, without excluding thatfluctuations in the doping concentration within the drift layer in theorder of a factor of one to five may be possible due to manufacturingreasons.

This design of the power diode of the invention has a segmentedstructure on the anode and on the cathode side. Specifically, it has asegmented second anode layer on the anode side and a cathode layer beingsegmented by the cathode-side segmentation layer. The segmentedstructure of the power diode of the invention improves the reverserecovery of the power diode for a minimum thickness of the power diode(throughout the specification a thickness of a power diode shall referto a shortest distance between anode and cathode electrode layers).Compared to the FCE diode which relies on injection of holes during thetail (end) phase of the reverse recovery period, characteristics of thepower diode of the invention are less temperature dependent. The softreverse recovery behavior (i.e., the significantly reduced voltage peakduring reverse recovery as explained in more detail below with referenceto FIG. 6) is, contrary to the FCE diode, not a result of injection ofcarriers from a p⁺ region at the cathode side but rather a result of thestored charge left in the unsegmented region.

In a first exemplary embodiment, in at least one vertical cross-sectionperpendicular to the horizontal plane, the second anode layers of eachpair of neighboring diode cells are laterally separated from each otherby the drift layers of the respective pair of neighboring diode cells.Throughout the specification, if a first region (or layer) is separatedfrom a second region (or layer) by a third region (or layer), this shallmean that there is no direct contact between the first and the secondregion but that there is continuous path from the first region to thesecond region through the third region without passing through any otherregion.

In the first exemplary embodiment, in the at least one verticalcross-section, the cathode-side segmentation layers of each pair ofneighboring diode cells may laterally be separated from each other bythe second conductivity type cathode layers of the respective pair ofneighboring diode cells. This can further improve the reverse recoveryof the power diode.

In the first exemplary embodiment, in the at least one verticalcross-section, a shortest lateral distance L_(d1) between the secondanode layers of each pair of neighboring diode cells may be in a rangefrom 0.3·L_(p1) to L_(p1), wherein L_(p1) is a smallest lateral width ofthe cathode-side segmentation layers in each one of the pair ofneighboring diode cells.

In the first exemplary embodiment, in the at least one verticalcross-section, a shortest lateral distance L_(n1) between thecathode-side segmentation layers of each pair of neighboring diode cellsmay be in a range from 0.3·W_(n) to W_(n), wherein W_(n) is a verticalthickness of the diode cells.

In the first exemplary embodiment, in the at least one verticalcross-section, a lateral width L_(p1) of the cathode-side segmentationlayer may be in a range from 0.3·W_(n) to W_(n), wherein W_(n) is avertical thickness of the diode cell.

In a second exemplary embodiment, in at least one vertical cross-sectionperpendicular to the horizontal plane, a portion of the drift layerlaterally separates the second anode layer in each diode cell into twoseparate regions laterally extending from the portion of the drift layerto an edge of the diode cell.

In the second exemplary embodiment, in the at least one verticalcross-section, the cathode layers of each pair of neighboring diodecells may be laterally separated from each other by the cathode-sidesegmentation layers of the respective pair of neighboring diode cells.

In the second exemplary embodiment, in the at least one verticalcross-section, a shortest lateral distance L_(d2) between the twoseparate regions of the second anode layer in each diode cell of eachpair of neighboring diode cells may be in a range from 0.3·L_(p2) toL_(p2), wherein L_(p2) is a shortest lateral distance between thecathode layers of the pair of neighboring diode cells in the at leastone vertical cross-section.

In the second exemplary embodiment, in the at least one verticalcross-section, a lateral width L_(n2) of the cathode layer of each diodecell may be in a range from 0.3·W_(n) to W_(n), wherein W_(n) is avertical thickness of the diode cell.

In the second exemplary embodiment, in the at least one verticalcross-section, a shortest lateral distance L_(p2) between the twocathode-side segmentation layers of each pair of neighboring diode cellsmay be in a range from 0.3·W_(n) to W_(n), wherein W_(n) is a verticalthickness of the diode cells.

In an exemplary embodiment, the plurality of diode cells all have thesame design or structure. The symmetry in a power diode according tosuch exemplary embodiment allows most homogeneous devicecharacteristics.

In an exemplary embodiment, the power diode has a honeycomb structure,wherein each diode cell has a hexagonal shape in horizontalcross-section. Alternatively, each diode cell may have a stripe shape inhorizontal cross-section.

In an exemplary embodiment the power diode comprises a secondconductivity type buffer layer. The buffer layer has a dopingconcentration higher than that of the drift layer and lower than that ofthe cathode layer. The buffer layer is separated from the cathodeelectrode layer by the cathode layer and by the cathode-sidesegmentation layer. The buffer layer is separated from the first anodelayer and from the second anode layer by the drift layer. In anexemplary embodiment a peak doping concentration of the buffer layer ishigher than 1·10¹⁶ cm⁻³ or higher than 2·10¹⁶ cm⁻³ or higher than 4·10¹⁶cm⁻³. Contrary to the FCE design, the concept of the invention isindependent of the buffer design i.e. the peak doping of the buffer isnot limited to values below a certain limit as it is the case for theFCE design. With a higher peak doping of the buffer layer, the powerdiode of the invention is less sensitive to temperature variations.

The power diode of the invention may be integrated together with agate-commutated thyristor (GCT) in a reverse conducting integratedgate-commutated thyristor (RC-IGCT) device. A soft reverse recoverybehavior of the device in diode mode is obtained with a minimumthickness of the device. Therefore, the efficiency of the device can beimproved both in diode- and GCT mode of operation while ensuring softrecovery behavior.

BRIEF DESCRIPTION OF THE DRAWINGS

Detailed embodiments of the invention will be explained below withreference to the accompanying figures, in which:

FIG. 1 illustrates time dependency of current and voltage during snappyrecovery of a conventional power diode;

FIG. 2 shows a top view of power diode according to a first, third andfourth embodiment;

FIG. 3 is a horizontal cross-section of nine diode cells included in thepower diode according to the first embodiment along line I-I′ in FIG. 5;

FIG. 4 is a horizontal cross-section of nine diode cells included in thepower diode according to the first embodiment along line II-II′ in FIG.5;

FIG. 5 shows a vertical cross-section of the power diode of the firstembodiment along any one of lines A-A′, B-B′ and C-C′ in FIGS. 3 and 4,respectively;

FIG. 6 shows a time dependency of current and voltage during a reverserecovery period for the power diode according to the first embodimentand for a conventional (unsegmented) power diode;

FIG. 7 shows a horizontal cross-section of a power diode according to asecond embodiment along line I-I′ in FIG. 9;

FIG. 8 shows a horizontal cross-section of a power diode according tothe second embodiment along line II-II′ in FIG. 9;

FIG. 9 shows a vertical cross-section of the power diode according tothe second embodiment along line E-E′ in FIGS. 7 and 8, respectively;

FIG. 10 shows a power diode according to the third embodiment incross-section along any one of lines A-A′, B-B′ and C-C′ in FIGS. 3 and4, respectively;

FIG. 11 shows a horizontal cross-section of nine diode cells included inthe power diode according to a fourth embodiment along line I-I′ in FIG.13;

FIG. 12 shows a horizontal cross-section of nine diode cells included inthe power diode according to the fourth embodiment along line II-II′ inFIG. 13;

FIG. 13 shows a vertical cross-section of the power diode according tothe fourth embodiment along any one of lines A-A′, B-B′ and C-C′ inFIGS. 11 and 12, respectively; and

FIG. 14 shows a top view of an RC-IGCT comprising the power diodeaccording to an embodiment of the invention.

The reference signs used in the figures and their meanings aresummarized in the list below. Generally, similar elements have the samereference signs throughout the specification. The described embodimentsare meant as examples and shall not limit the scope of the invention.Similar reference signs comprising the same reference numeral but havingdifferent number of dashes (e.g. reference signs 100, 100′, 100″, 100′″)refer to similar elements/entities in different embodiments. Thedescription of features for one of these similar reference signs shallapply to all elements/entities referenced by these reference signs,except where it is describe otherwise.

5 edge termination 10, 10′, 10″, 10″′ diode cell 20 anode electrodelayer 30 cathode electrode layer 40, 40″ first anode layer 45, 45′, 45″,45″′ second anode layer 50, 50′, 50″, 50″′ drift layer 60, 60′, 60″′cathode layer 67, 67′, 67″′ cathode-side segmentation layer 80insulation layer 90 RC-IGCT 91, 91′, 91″, 91″′, 910 power diode 92separation region 93 gate commutated thyristor (GCT) 94 gate contact100, 100′, 100″, 100″′ semiconductor wafer 101, 101′, 101″, 101″′ firstmain side 102, 102′, 102″, 102″′ second main side 110 incomplete cellK1, K1′, K1″, K1″′ first horizontal plane K2, K2′, K2″, K2″′ secondhorizontal plane Ld1 shortest lateral distance (between second anodelayers of two neighboring diode cells) Ln1 shortest lateral distance(between cathode-side segmentation layers of two neighboring diodecells) Lp1 lateral width (of cathode-side segmentation layer) Ld2shortest lateral distance (between two separate regions of the secondanode layer) Ln2 lateral width (of cathode-side segmentation layer) Lp2shortest lateral distance (between cathode layers of two neighboringdiode cells Wn vertical thickness of the diode cells

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following there is described a power diode 91 according to afirst embodiment of the invention with reference to FIGS. 2 to 6 (thereference signs in parentheses relate to other embodiments discussed indetail further below). FIG. 2 shows the power diode 91 in top view, FIG.3 is a horizontal cross-section of nine diode cells 10 included in thepower diode 91 along line I-I′ in FIG. 5, FIG. 4 is a horizontalcross-section of nine diode cells 10 included in the power diode 91along line II-II′ in FIG. 5, and FIG. 5 shows a cross-section of thepower diode 91 along any one of lines A-A′, B-B′ and C-C′ in FIGS. 3 and4, respectively (in the power semiconductor device 91 the cross-sectionalong line A-A′ is the same as the cross-section along line B-B′ and isalso the same as the cross-section along line C-C′).

The power diode 91 comprises a semiconductor wafer 100 having a firstmain side 101 and a second main side 102 as shown in FIG. 5. As can beseen from FIG. 2, the semiconductor wafer 100 includes an active regionlaterally surrounded by an edge termination region 5. In the activeregion the power diode 91 comprises a plurality of diode cells 10.Except for incomplete cells 110 which are directly adjacent to the edgetermination region 5, all diode cells 10 may have the same structure,which is exemplarily a hexagonal structure. The hexagonal structure ischaracterized by a hexagonal outer shape in top view (i.e. in a view ina direction orthogonal to the first main side 101, which is a verticaldirection). The diode cells 10 having the hexagonal structure form ahoneycomb pattern in top view as shown in FIG. 2. Incomplete cells 110directly adjacent to the edge termination region 5 may have, compared tothe diode cells 10 which are not directly adjacent to the edgetermination region 5, a different structure which includes only a partof the hexagonal structure of the diode cells 10. In the horizontalcross-sections of FIGS. 3 and 4 nine neighboring diode cells 10 havingthe same hexagonal structure are shown respectively.

In FIGS. 2 to 4 solid lines represent the boundaries between neighboringdiode cells 10, whereas thin dashed lines in FIG. 3 illustrate theboundaries (i.e. the outer edges or peripheries) of p-type second anodelayers 45 in a first horizontal plane K1 along line I-I′ in FIG. 5 andthin dashed lines in FIG. 4 illustrate the boundaries of cathode-sidesegmentation layers 67 in a second horizontal plane K2 along line II-II′in FIG. 5. As shown in FIGS. 3 and 4 the boundaries of the p-type secondanode layers 45 and the boundaries of the cathode-side segmentationlayers 67 may be hexagonal similar to the outer shape of the diode cells10 in top view.

FIG. 5 shows three neighboring (i.e. directly adjacent) diode cells 10in cross-section along any one of lines A-A′, B-B′ and C-C′ in FIGS. 3and 4, respectively. As shown in FIG. 5 the power diode 91 comprises ananode electrode layer 20 formed on the first main side 101 of thesemiconductor wafer 100 and a cathode electrode layer 30 formed on thesecond main side 102 of the semiconductor wafer 100. The anode electrodelayer 20 and the cathode electrode layer 30 cover at least a portion ofeach diode cell 10. Exemplarily, the anode electrode layer 20 and thecathode electrode layer 30 may respectively be formed as a continuousmetallization layer on the first main side 101 and on the second mainside 102, respectively.

Accordingly, the diode cells 10 in the semiconductor wafer 100 arearranged between the anode electrode layer 20 and the cathode electrodelayer 30. Each diode cell 10 includes a p-type first anode layer 40which is in direct contact with the anode electrode layer 20, the secondanode layer 45 which is separated from the anode electrode layer 20 bythe first anode layer 40 (along any straight vertical line from theanode electrode layer 20 to the cathode electrode layer 30, wherein avertical direction is a direction from the anode electrode 20 to thecathode electrode layer 30 along a shortest line connecting the anodeelectrode 20 with the cathode electrode layer 30), an n-type drift layer50 forming a pn-junction with the second anode layer 45, an n-typebuffer layer 65, an n-type cathode layer 60 being in direct contact withthe cathode electrode layer 30, and a cathode-side segmentation layer 67being in direct contact with the cathode electrode layer 30 andalternating with the cathode-side segmentation layer 67. The bufferlayer 65 is separated from the first anode layer 40 and from the secondanode layer 45 by the drift layer 50, and the buffer layer 65 isseparated from the cathode electrode layer 30 by the cathode layer 60and the cathode-side segmentation layer 67. The second anode layer 45may be a well region within the drift layer 50.

The power diode 91 according to the first embodiment may be a siliconbased power diode, i.e. semiconductor wafer 100 including the firstanode layer 40, the second anode layer 45, the n-type drift layer 50,the n-type buffer layer 65 and the n-type cathode layer 60 may be madeof silicon.

The first anode layer 40 in the power diode 91 according to the firstembodiment is a continuous layer shared by the plurality of diode cells10. In all diode cells 10 of the plurality of diode cells 10 the firstanode layer 40 may have a constant thickness d1 in a directionperpendicular to the first main side 101, i.e. the first anode layer 40extends in the vertical direction from the surface 101 of thesemiconductor wafer 100 to a first depth d1. The second anode layer 45extends in the vertical direction from the first anode layer 40 to asecond depth d2.

The second anode layers 45 of each pair of neighboring diode cells 10are laterally separated from each other by portions of the n-type driftlayer 50 of the respective pair of neighboring diode cells 10. Morespecifically, the second anode layer 45 of each diode cell 10 islaterally surrounded and separated from the periphery of the diode cell10 by a portion of the drift layer 50 as shown in FIGS. 3 and 5.Throughout the specification a lateral direction shall refer to adirection parallel to the first main side 101 or the second main side102. Accordingly, the second anode layer 45 of each diode cell 10 islaterally surrounded and separated from the periphery of the diode cell10 by a portion of the drift layer 50 in in an orthogonal projectiononto a plane parallel to the first main side 101 (or parallel to thesecond main side 102). Accordingly, the horizontal cross-section througheach diode cell 10 along the first horizontal plane K1 perpendicular tothe vertical direction at a third depth d3 comprises in each diode cell10 a first area where the first horizontal plane K1 intersects thesecond anode layer 45 and a second area where the first horizontal planeK1 intersects the drift layer 50, wherein the third depth d3 is a depthbetween the depth d1 to which the first anode layer 40 extends and thesecond depth d2 to which the second anode layer 45 extends.

In the vertical cross-section shown in FIG. 5 the shortest lateraldistance between two second anode layers 45 of each pair of neighboringdiode cells 10 is referred to as L_(d1). Due to the identity of thestructure or design of the diode cells 10 in the power diode 91, adistance between the anode layer 45 of a diode cell 10 to the peripheryof that diode cell 10 is L_(d1)/2, and the distance L_(d1) between twoneighboring second anode layers 45 in the cross-section shown in FIG. 5is the same for all pairs of directly adjacent diode cells 10.

Accordingly, a p-type region comprising the first anode layer 40 and thesecond anode layer 45 has the first depth d1 at the periphery of thediode cell 10 and has the second depth d2 in the lateral center of thediode cell 10, wherein the second depth d2 is larger than the firstdepth d1. Exemplarily a thickness of the first anode layer 40 in adirection perpendicular to the first main side 101 is in a range between2 μm to 80 μm, i.e. the first depth d1 is in a range between 2 μm and 80μm. A thickness of the second anode layer 45 in the directionperpendicular to the first main side 101 is exemplarily in a rangebetween 50 μm and 200 μm, i.e. a difference d2−d1 between the seconddepth d2 and the first depth d1 is in a range between 50 μm and 200 μm.The first anode layer 40 comprises a first doping concentration of afirst p-type dopant and the second anode layer 45 comprises a seconddoping concentration of a second p-type dopant, which has a lowersurface concentration than the first p-type dopant. Exemplarily, thefirst p-type dopant is different from the second p-type dopant, forexample the first p-type dopant may boron (B) and the second p-typedopant may be aluminum (Al).

A peak doping concentration of the first anode layer 40 is higher than apeak doping concentration of the second anode layer 45. Exemplarily, thepeak doping concentration of the first anode layer 40 is above 5·10¹⁷cm⁻³ and the peak doping concentration of the second anode layer 45 isbelow 5·10¹⁷ cm⁻³.

The drift layer 50 is shared by all diode cells 10 and extends in anorthogonal projection onto a plane parallel to the first main side 101in the whole area of each diode cell 10, i.e. the drift layer 50 extendslaterally through each diode cell 10. The drift layer 50 forms apn-junction with the first anode layer 40 in areas where the first anodelayer 40 is not overlapped with the second anode layer 45 in theorthogonal projection onto the plane parallel to the first main side101. In areas where the first anode layer 40 is overlapped with thesecond anode layer 45 in the orthogonal projection onto the planeparallel to the first main side 101, the drift layer 50 forms apn-junction with the second anode layer 45. The thickness of the driftlayer 50 depends on the voltage class of the power diode. A dopingconcentration of the drift layer 50 is relatively low (low compared tothe doping concentration exemplarily of the other layers like the bufferlayer 65), exemplarily below 5·10¹³ cm⁻³ depending on the voltage classof the power diode 91. The drift layer 50 may have a constant dopingconcentration. Therein, a constant doping concentration means that thedoping concentration is substantially homogeneous throughout the driftlayer 50, however without excluding that fluctuations in the dopingconcentration within the drift layer 50 in the order of a factor of oneto five may be possible due to manufacturing reasons. A dopingconcentration of the buffer layer 65 is higher than that of the driftlayer 50. Exemplarily, the buffer layer 65 may have a rising dopingconcentration towards the second main side 102. A peak dopingconcentration of the buffer layer 65 is exemplarily higher than 1·10¹⁶cm⁻³, exemplarily higher than 2·10¹⁶ cm⁻³ or more exemplarily higherthan 4·10¹⁶ cm⁻³. In the horizontal cross-section as shown in FIG. 4,the cathode-side segmentation layer 67 is located in the lateral centerof the diode cell 10 surrounded by the n-type cathode layer 60.Accordingly, in top view (orthogonal projection onto a horizontal plane)the second anode layer 45 fully overlaps the cathode-side segmentationlayer 67.

A material of the cathode-side segmentation layer 67 is either a p-typesemiconductor or an electrically insulating material, such as siliconoxide or oxynitride. The material of the cathode-side segmentation layer67 may be any material that can inhibit electron emission from thebuffer layer 65 into the drift region during diode conduction in an areaadjacent to or above the cathode-side segmentation layer 67. Thethickness of the cathode-side segmentation layer 67 is less than athickness of the highly n-type doped cathode layer 60. The dopingconcentration of the cathode layer 60 is significantly higher than thatof the buffer layer and may be for example above 10¹⁸ cm⁻³.

Contrary to the known FCE diode, which requires significant holeinjection to soften the collapse of the current as the electric fieldsweeps out the last charge close to the buffer (i.e. the FCE diode needsa strong p-emitter), the segmented power diode 91 of the invention doesnot require any injection of holes from the cathode side. Quite to thecontrary, FCE action, i.e. significant injection of holes, is notdesirable because it is temperature dependent and gives the strongesteffect at high temperatures where it is least needed. In case that thesegmentation layer 67 is made of an insulating material there is noinjection of holes. On the other side, in case that the cathode-sidesegmentation layer 67 is made of a p-type semiconductor material, it isdesirable that the emitter efficiency is relatively low. The emitterefficiency of a p-type cathode-side segmentation layer 67 dependsbasically on the doping concentration of the cathode-side segmentationlayer 67 and on its depth (i.e. thickness in a direction perpendicularto second main side 102). The dose or integrated doping content(integrated along a direction perpendicular to the second main side 102)of the cathode-side segmentation layer 67 is below 2·10¹³ cm⁻² orexemplarily below 1·10¹³ cm⁻² or more exemplarily below 5·10¹² cm⁻².Therein the doping content refers to an activated dopant. The lower thep-type dose the lower the emitter efficiency of the p-type cathode-sidesegmentation layer 67 for injection of holes. Employing a p-typematerial for the cathode segmentation layer 67 may facilitate themanufacturing of the power diode compared to a case where an insulatingmaterial is used for the cathode segmentation layer 67.

Like the second anode layers 45 also the cathode-side segmentationlayers 67 are segmented. The cathode-side segmentation layers 67 of eachpair of neighboring diode cells 10 are laterally separated from eachother by the n-type cathode layers 60 of the respective pair ofneighboring diode cells 10. More specifically, as can be seen from thevertical cross-section shown in FIG. 5 and the horizontal cross-sectionshown in FIG. 4, the cathode-side segmentation layer 67 of each diodecell 10 is laterally (i.e. in the orthogonal projection onto a planeparallel to the first main side 101 or the second main side 102)surrounded and separated from the periphery of the diode cell 10 by aportion of the n-type cathode layer 60. A shortest lateral distancebetween two cathode-side segmentation layers 67 of each pair ofneighboring diode cells 10 is referred to as L_(n1) in the verticalcross-section shown in FIG. 5. Due to the identity of the structure ordesign of the diode cells 10, the distance between the cathode-sidesegmentation layer 67 of a diode cell 10 to the periphery of that diodecell 10 is L_(n1)/2. A lateral width of the cathode-side segmentationlayer 67 of a diode cell 10 is referred to as L_(p1) in the verticalcross-section shown in FIG. 5.

Relationships (design rules) between the lateral width L_(p1) of thecathode-side segmentation layer 67 of each diode cell, the shortestlateral distance L_(n1) between two cathode-side segmentation layers 67of each pair of neighboring diode cells 10, the shortest lateraldistance L_(d1) between the second p-type layers 45 of each pair ofneighboring diode cells 10 and the thickness W_(n1) of each diode cell10 may be the following:

0.3·L _(p1) ≤L _(d1) ≤L _(p1)  (i)

0.3·W _(n1) ≤L _(n1) ≤W _(n1)  (ii)

0.3·W _(n1) ≤L _(p1) ≤W _(n1)  (iii)

In FIG. 6 there is shown a time dependency of current and voltage duringa reverse recovery period for the power diode 91 according to the firstembodiment and for a conventional unsegmented power diode (conventionalPiN diode) as a comparison. The solid lines in the graph of FIG. 6correspond to the current and voltage of the power diode 91 according tothe first embodiment, and the dashed lines correspond to the current andvoltage of the conventional unsegmented power diode. As can be seenclearly from FIG. 6 the power diode 91 of the invention shows a softrecovery behavior whereas the conventional power diode shows a snappyreverse recovery behavior with pronounced oscillations of the voltage.

In the following a power diode 91′ according to a second embodiment willbe explained with reference to FIGS. 5, and 7 to 9. Due to manysimilarities between the power diode 91 according to the firstembodiment and the power diode 91′ according to the second embodiment,not all features which are the same for the two power diodes 91 and 91′according to the first and the second embodiment will be repeated in thefollowing. The description of the power diode 91′ according to thesecond embodiment will focus on the differences between the first andthe second embodiments, whereas with all other features it is referredto the description of the first embodiment above. In particular, samereference signs in the drawings refer to similar elements that have thesame characteristics or features.

The power diode 91′ according to the second embodiment differs from thepower diode 91 according to the first embodiment in that each diode cell10′ of the power diode 91′ has a stripe shape in horizontalcross-section. This can be seen in FIG. 7 which shows three diode cells10′ of the power diode 91′ in a first horizontal cross-section alongline I-I′ in FIGS. 5 and 9, and in FIG. 8 which shows these three diodecells 10′ in a second horizontal cross-section along line II-II′ inFIGS. 5 and 9, respectively. FIG. 5 shows a first vertical cross-sectionof these three diode cells 10′ along line D-D′ in FIGS. 7 and 8,respectively. Accordingly, the cross-section of diode cells 10′ alongline D-D′ in FIGS. 7 and 8 looks the same as the cross-section of thediode cells 10 along any one of lines A-A′, B-B′ and C-C′ in FIGS. 3 and4, respectively. Reference signs related exclusively to the secondembodiment are shown in FIG. 5 in parentheses. In particular thecross-sections of the stripe shaped p-type second anode layers 45′ andof the stripe shaped cathode-side segmentation layers 67′ along lineD-D′ are the same as the cross-sections of second anode layer 45 and ofthe cathode-side segmentation layers 67 along lines A-A′, B-B′, and C-C′in FIGS. 3 and 4, respectively.

In the second embodiment, a semiconductor wafer 100′ a partial verticalcross-section of which is shown in FIG. 5 has a first main side 101″ anda second main side 102″ and is similar to the semiconductor wafer 100described above. It differs from the above described semiconductor wafer100 only in the different shape of the second anode layer 45′, the driftlayer 50′, the buffer layer 65′, the cathode layer 60′ and of thecathode segmentation layer 67′ due to the above described stripe-shapeof the diode cells 10′. Like the power diode 91, the power diode 91′ mayhave more than three diode cells 10′. In addition, the power diode 91′may have an edge termination region 5 and incomplete diode cells 110adjacent to the edge termination region as described above for the firstembodiment. In the horizontal cross-sections of FIGS. 7 and 8 the solidlines represent the boundaries between neighboring diode cells 10′,whereas thin dashed lines in FIG. 7 illustrate the boundaries of secondanode layers 45′ in a first horizontal plane K1‘along line I-I′ in FIGS.5 and 9, and thin dashed lines in FIG. 8 illustrate the boundaries ofcathode-side segmentation layers 67′ in a second horizontal plane K2′along line II-II′ in FIGS. 5 and 9. In top view, i.e. in an orthogonalprojection onto a plane perpendicular to the vertical direction, as wellas in the first and second horizontal cross-sections shown in FIGS. 7and 8, each one of the second anode layers 45′ and of the cathode-sidesegmentation layers 67′ has a stripe shape. A longitudinal axis of thestripe shaped second anode layers 45′ and of the stripe shapedcathode-side segmentation layers 67′ extends along line E-E′ in FIGS. 7and 8, i.e. in the left-right direction in FIG. 9, throughout the wholediode cell 10′. In the horizontal cross-section of FIG. 7 stripe-shapedregions of the drift layer 50′ alternate with stripe-shaped regions ofthe second anode layers 45′. Likewise, in the horizontal cross-sectionshown in FIG. 8 stripe shaped regions of the cathode layers 60′ withstripe-shaped regions of the cathode segmentation layers 67″.

Similar to the above described first embodiment, in the verticalcross-section shown in FIG. 5, a shortest lateral distance L_(d1)between two second anode layers 45′ of each pair of neighboring diodecells 10′ (which are neighboring in a direction parallel to line D-D′ inFIGS. 7 and 8, respectively), a shortest lateral distance L_(n1) betweentwo cathode-side segmentation layers 67′ of each pair of neighboringdiode cells 10′, a lateral width L_(p1) of each cathode-sidesegmentation layer 67′ in a direction parallel to line D-D′ in FIGS. 7and 8, respectively, and the thickness W_(n) of each diode cell 10′ mayexemplarily fulfil the above indicated design rules (i) to (iii).

In the following a power diode 91″ according to a third embodiment willbe explained with reference to FIGS. 2 to 4, and 10. Therein, FIG. 3shows a horizontal cross-section of diode cells 10″ of the power diode91″ along line I-I′ in FIG. 10 (along plane K1″ in FIG. 10), and FIG. 4shows another horizontal cross-section of these diode cells 10″ alongII-II″ in FIG. 10 (i.e. along plane K2″ in FIG. 10). Due to manysimilarities between the power diode 91 according to the firstembodiment and the power diode 91″ according to the third embodiment notall features, which are the same for the two power diodes 91 and 91″,will be repeated in the following. The description of the power diode91″ according to the third embodiment will focus on the differencesbetween the first embodiment and the third embodiment, whereas with allother features it is referred to the description of the first embodimentabove. In particular, same reference signs in the drawings refer tosimilar elements that have the same characteristics or features.

The power diode 91″ of the third embodiment differs from the power diode91 according to the first embodiment as described above with referenceto FIGS. 2 to 5 only in a different shape of the p-type first anodelayer 40″ and of the p-type second anode layer 45″. In the thirdembodiment the first anode layer 40″ is configured as a well regionwithin the second anode layer 45″. The top view of the power diode 91″according to the third embodiment is the same as the top view of thepower diode 91 according to the first embodiment shown in FIG. 2.Reference signs related exclusively to the third embodiment are shown inFIGS. 2, 3 and 4 in parentheses. A cross-section of the power diode 91″along any one of lines A-A′, B-B′, and C-C′ in FIGS. 3 and 4, is shownin FIG. 10. Due to the modified shape of the first anode layer 40″ andof the second anode layer 45″, a drift layer 50″ differs from the driftlayer 50 in the first embodiment in that it is not in direct contactwith but is separated from the first anode layer 40″ by the second anodelayer 45″.

In the third embodiment, a semiconductor wafer 100″ shown in FIG. 10having a first main side 101″ and a second main side 102″ is similar tothe semiconductor wafer 100 described above. It differs from the abovedescribed semiconductor wafer 100 only in the different configuration ofthe first anode layers 40″, the second anode layers 45″ and the driftlayer 50″. Contrary to the drift layer 50 in the first embodiment, inthe third embodiment the drift layer 50″ extends up to the first mainside 101″ of semiconductor wafer 100″. Likewise, the second anode layer45″ extends up to the first main side 101″. An Insulation layer 80provided on the first main side 101″ to cover the drift layer 50″prevents the anode electrode 20 from directly contacting the drift layer50″ and from directly contacting the second anode layer 45″.

In the following a power diode 91′″ according to a fourth embodimentwill be explained with reference to FIGS. 2 and 11 to 13. Due to manysimilarities between the power diode 91 according to the firstembodiment and the power diode 91′″ according to the fourth embodiment,not all features which are the same for the two power diodes 91 and 91′″according to the first and the fourth embodiment will be repeated in thefollowing. The description of the power diode 91′″ will focus on thedifferences between the first and the fourth embodiments, whereas withall other features it is referred to the description of the firstembodiment above. In particular, same or similar reference signs in thedrawings refer to similar elements that have the same characteristics orfeatures.

FIG. 2 shows the power diode 91′″ in top view. That means the top viewof power diode 91′″ is the same as the top view of the power diode 91according to the first embodiment. FIG. 11 is a first horizontalcross-section of nine diode cells 10′″ included in the power diode 91′″along line I-I′ in FIG. 13 (i.e. along horizontal plane K1′″ in FIG.13), FIG. 12 shows a second horizontal cross-section of nine diode cells10′″ included in the power diode 91′″ along line II-II′ in FIG. 13 (i.e.along horizontal plane K2′″ in FIG. 13), and FIG. 13 shows across-section of the power diode 91 along any one of lines A-A′, B-B′and C-C′ in FIGS. 11 and 12, respectively (in the power semiconductordevice 91′″ the cross-section along line A-A′ is the same as thecross-section along line B-B′ and is also the same as the cross-sectionalong line C-C′ in FIGS. 11 and 12).

The power diode 91′″ differs from the power diode 91 in that the secondanode layer 45′″ is not arranged in the lateral center of each diodecell 10′″ as it is the case in the first embodiment but is arrangedalong the outer boundary of each diode cell 10′″ in an orthogonalprojection onto a horizontal plane perpendicular to the verticaldirection. Accordingly, in the vertical cross-section perpendicular tothe first horizontal plane K1′″ as shown in FIG. 13, a portion of thedrift layer 50′″ laterally separates the second anode layer 45′″ in eachdiode cell 10′″ into two separate regions laterally extending from theportion of the drift layer 50′″ to an edge of the diode cell 10′″,respectively. In the first horizontal cross-section shown in FIG. 11,the before mentioned portion of the drift layer 50′″ in each diode cell10′″ is an island-like area (where the first horizontal plane K1′″intersects the portion of the drift layer 50′″) surrounded by an areawhere the first horizontal plane K1′″ intersects the second anode layer45′″.

The power diode 91′″ differs from the power diode 91 also in that thecathode layer 60′″ is not arranged in the lateral center of each diodecell 10′″ as it is the case in the first embodiment but is arrangedalong the outer boundary of each diode cell 10′″ in the horizontalcross-section shown in FIG. 12 and in an orthogonal projection onto ahorizontal plane perpendicular to the vertical direction. In thehorizontal cross-section shown in FIG. 12, the cathode segmentationlayer 67′″ in each diode cell 10′″ is an island-like area (where thesecond horizontal plane K2′″ intersects the segmentation layer 67′″)surrounded by an area where the second horizontal plane K2′″ intersectsthe cathode layer 60′″. In the vertical cross-section perpendicular tothe first horizontal plane K1′″ the cathode layers 60′″ of each pair ofneighboring diode cells 10′″ are laterally separated from each other bythe cathode-side segmentation layers 67′″ of the respective pair ofneighboring diode cells 10′″.

In the vertical cross-section as shown in FIG. 13, a shortest lateraldistance L_(d2) between the two separate regions of the second anodelayer 45′″ in each diode cell 10′″ of each pair of neighboring diodecells 10′″ is in a range from 0.3·L_(p2) to L_(p2), wherein L_(p2) is ashortest lateral distance between the cathode layers 60′″ of the pair ofneighboring diode cells 10′″. This feature corresponds to the abovedesign rule (i) in the first embodiment.

Also, in the vertical cross-section as shown in FIG. 13 a lateral widthL_(n2) of the cathode layer 60′″ of each diode cell 10′″ is in a rangefrom 0.3·W_(n) to W_(n), wherein W_(n) is the vertical thickness of thediode cell 10′″. This feature corresponds to the above design rule (ii).

Finally, in the vertical cross-section as shown in FIG. 13 a shortestlateral distance L_(p2) between the two cathode layers 60′″ of each pairof neighboring diode cells 10′″ is in a range from 0.3·W_(n) to W_(n),wherein W_(n) is the vertical thickness of the diode cells 10′″. Thisfeature corresponds to the above design rule (iii).

In the fourth embodiment, a semiconductor wafer 100′″ a partial verticalcross-section of which is shown in FIG. 13 has a first main side 101′″and a second main side 102′″ and is similar to the semiconductor wafer100 described above. It differs from the above described semiconductorwafer 100 only in the different shape and arrangement of the secondanode layer 45′″, the drift layer 50′″, the buffer layer 65′″, thecathode layer 60′″ and of the cathode segmentation layer 67′″ due to theabove described structure of the diode cells 10′″.

FIG. 14 shows a top view of a reverse conducting integratedgate-commutated thyristor (RC-IGCT) 90 according to an embodiment of theinvention. The RC-IGCT 90 comprises a gate-commutated thyristor (GCT) 93and a power diode 910 (acting as a freewheeling diode) monolithicallyintegrated in a single wafer. The GCT 93 is separated from the powerdiode 910 by a separation region 92 in the wafer. As illustrated in FIG.9, the GCT 93 comprises plural GCT fingers arranged in two ringssurrounding the power diode 91′″. Along the periphery of the wafer, theRC-IGCT has a gate contact 94 for control of the GCT 93 as shown in FIG.9. The design of an RC-IGCT as such is well known to the person skilledin the art. Therefore, it is refrained from a detailed description ofthe structure of the GCT 93 and the separation region 92. The RC-IGCT 90according to an embodiment of the invention differs from such well knownRC-IGCT only in that the power diode 910 is a power diode according tothe invention, for example structured as described above for any one ofthe above described power diodes 91, 91′, 91″ or 91′″ without the edgetermination region 5. It is to be mentioned that the new diode design inthe RC-IGCT 90 can be manufactured without the need of any additionalmask compared to the known manufacturing method for an RC-IGCT with aconventional PiN power diode as a freewheeling diode.

It will be apparent for persons skilled in the art that modifications ofthe above described embodiments are possible without departing from thescope of the invention as defined by the appended claims.

In the above first to third embodiment of a power diode, the shape ofthe diode cells 10, 10″ and 10′″ was described to be hexagonal in topview, and the shape of the diode cells 10′ was described to be stripeshaped. However, the diode cells 10, 10′, 10″, 10′″ may have any othershape such as a square shape or a triangular shape in top view, i.e. ina horizontal projection onto a plane parallel to the first main side101, 101′, 101″, 101′″. Likewise the outer shape of the second anodelayers 45, 45′, 45″, 45′″ in the power diode 91, 91′, 91″, 91′″ may haveanother shape than hexagonal or stripe shape, such as a square shape, atriangular shape, any other polygonal shape, or a circular shape in topview. Also, while in the first to third embodiments the outer shape ofthe diode cells 10, 10′, 10″, 10′″ is described to be the same as theouter shape of the second anode layers 45, 45′, 45″, 45′″ in top view(either hexagonal or stripe shaped), the outer shape of the diode cells10, 10′, 10″, 10′″ in the power diode 91, 91′, 91″, 91′″ of theinvention is not necessarily the same as the outer shape of the secondanode layers 45, 45′, 45″, 45′″ in top view.

In the above described embodiments, the diode cells 10, 10′, 10″, 10′″in one power diode 91, 91′, 91″, 91′″ had all the same design except theincomplete diode cells 110 directly adjacent to an edge terminationregion 5 or adjacent to the separation region 92 in the RC-IGCT 90.However, the power diode of the invention may employ diode cells havingtwo or more different designs among the plural diode cells, e.g.different sized diode cells.

In the RC-IGCT the gate contact 94 is described to be located at theperiphery of the device surrounding the GCT 93. However, the gatecontact 94 may also be located at another location such as between tworings of thyristor fingers. Also, the GCT 93 may have any otherarrangement of GCT fingers, and may have in particular any other numberof rings in which the thyristor fingers are arranged.

In the above described embodiments, the power diode of the invention wasdescribed to be either a discrete device as in the first to thirdembodiments or to be integrated in an RC-IGCT. However, the power diodeof the invention may be employed or integrated in any other powerdevice, such as in combination with insulated-gate bipolar transistors(IGBTs) and gate turn-off thyristors (GTOs) as a freewheeling diode,snubber diode and clamp diode, for example.

In the above embodiments the power diode of the invention was describedto include a buffer layer i.e. to have a punch-through (PT)configuration. However, the above embodiments may also be modified tohave no buffer layer, i.e. to have a non-punch-through (NPT)configuration.

The embodiments were explained with specific conductivity types. Theconductivity types of the semiconductor layers in each of theabove-described embodiments may be switched, so that all layers whichare described as p-type layers would be n-type layers and all layerswhich are described as n-type layers would be p-type layers.

It should be noted that the term “comprising” does not exclude otherelements or steps and that the indefinite article “a” or “an” does notexclude the plural. Also, elements described in association withdifferent embodiments may be combined.

1-15. (canceled)
 16. A power diode comprising an anode electrode layer, a cathode electrode layer and a plurality of diode cells arranged between the anode electrode layer and the cathode electrode layer, wherein each diode cell comprises: a first anode layer of a first conductivity type in direct contact with the anode electrode layer and extending in a vertical direction from the anode electrode layer to a first depth, the vertical direction being defined as a direction from the anode electrode layer to the cathode electrode layer; a second anode layer of the first conductivity type having a lower doping concentration than the first anode layer and being separated from the anode electrode layer by the first anode layer, the second anode layer extending in the vertical direction from the first anode layer to a second depth that is larger than the first depth; a drift layer of a second conductivity type forming a pn-junction with the second anode layer, wherein the second conductivity type is different from the first conductivity type and wherein a horizontal cross-section through each diode cell along a horizontal plane perpendicular to the vertical direction at a third depth comprises, in each diode cell, a first area where the horizontal plane intersects the second anode layer and a second area where the horizontal plane intersects the drift layer, the first depth being smaller than the third depth and the third depth being smaller than the second depth; a cathode layer of second conductivity type in direct contact with the cathode electrode layer, the cathode layer having a higher doping concentration than the drift layer; and a cathode-side segmentation layer in direct contact with the cathode electrode layer, wherein a material of the cathode-side segmentation layer is an insulating material or a lightly doped semiconductor of the first conductivity type having an integrated doping content along a direction perpendicular to a main side that is below 2·10¹³ cm⁻².
 17. The power diode according to claim 16, wherein the material of the cathode-side segmentation layer is an insulating material.
 18. The power diode according to claim 16, wherein the material of the cathode-side segmentation layer is a lightly doped semiconductor of the first conductivity type having a integrated doping content along a direction perpendicular to the main side that is below 2·10¹³ cm⁻².
 19. The power diode according to claim 16, wherein all diode cells of the plurality of diode cells have the same structure.
 20. The power diode according to claim 16, wherein each diode cell has a hexagonal shape.
 21. The power diode according to claim 16, wherein each diode cell has a stripe shape in a horizontal cross-section.
 22. The power diode according to claim 16, further comprising a buffer layer of the second conductivity type, wherein: the buffer layer has a higher doping concentration than the drift layer; the cathode layer has a higher doping concentration than that of the buffer layer; the buffer layer is separated from the cathode electrode layer by the cathode layer and by the cathode-side segmentation layer; and the buffer layer is separated from the first anode layer and from the second anode layer by the drift layer.
 23. A reverse conducting integrated gate-commutated thyristor device comprising: a gate-commutated thyristor; and the power diode according to claim
 16. 24. A power diode comprising an anode electrode layer, a cathode electrode layer and a plurality of diode cells arranged between the anode electrode layer and the cathode electrode layer, wherein each diode cell comprises: a first anode layer of a first conductivity type in direct contact with the anode electrode layer and extending in a vertical direction from the anode electrode layer to a first depth, the vertical direction being defined as a direction from the anode electrode layer to the cathode electrode layer; a second anode layer of the first conductivity type having a lower doping concentration than the first anode layer and being separated from the anode electrode layer by the first anode layer, the second anode layer extending in the vertical direction from the first anode layer to a second depth that is larger than the first depth; a drift layer of a second conductivity type forming a pn-junction with the second anode layer, wherein the second conductivity type is different from the first conductivity type and wherein a horizontal cross-section through each diode cell along a horizontal plane perpendicular to the vertical direction at a third depth comprises, in each diode cell, a first area where the horizontal plane intersects the second anode layer and a second area where the horizontal plane intersects the drift layer, the first depth being smaller than the third depth and the third depth being smaller than the second depth and wherein, in a vertical cross-section perpendicular to the horizontal plane, the second anode layers of each pair of neighboring diode cells are laterally separated from each other by the drift layers of the respective pair of neighboring diode cells; a cathode layer of second conductivity type in direct contact with the cathode electrode layer, the cathode layer having a higher doping concentration than the drift layer; and a cathode-side segmentation layer in direct contact with the cathode electrode layer, wherein a material of the cathode-side segmentation layer is an insulating material or a lightly doped semiconductor of the first conductivity type having an integrated doping content along a direction perpendicular to a main side that is below 2·10¹³ cm⁻².
 25. The power diode according to claim 24, wherein the material of the cathode-side segmentation layer is an insulating material.
 26. The power diode according to claim 24, wherein the material of the cathode-side segmentation layer is a lightly doped semiconductor of the first conductivity type having a integrated doping content along a direction perpendicular to the main side that is below 2·10¹³ cm⁻².
 27. The power diode according to claim 24, wherein, in the vertical cross-section, the cathode-side segmentation layers of each pair of neighboring diode cells are laterally separated from each other by the cathode layers of the respective pair of neighboring diode cells.
 28. The power diode according to claim 24, wherein, in the vertical cross-section, a shortest lateral distance L_(d1) between the second anode layers of each pair of neighboring diode cells is in a range from 0.3·L_(p1) to L_(p1), wherein L_(p1) is a lateral width of the cathode-side segmentation layers of each one of the pair of neighboring diode cells in the vertical cross-section.
 29. The power diode according to claim 24, wherein, in the vertical cross-section, a shortest lateral distance L_(n1) between the cathode-side segmentation layers of each pair of neighboring diode cells is in a range from 0.3·W_(n) to W_(n), wherein W_(n) is a vertical thickness of the diode cells.
 30. The power diode according to claim 24, wherein, in the vertical cross-section, a lateral width L_(p1) of the cathode-side segmentation layer is in a range from 0.3·W_(n) to W_(n), wherein W_(n) is a vertical thickness of the diode cells.
 31. A power diode comprising an anode electrode layer, a cathode electrode layer and a plurality of diode cells arranged between the anode electrode layer and the cathode electrode layer, wherein each diode cell comprises: a first anode layer of a first conductivity type in direct contact with the anode electrode layer and extending in a vertical direction from the anode electrode layer to a first depth, the vertical direction being defined as a direction from the anode electrode layer to the cathode electrode layer; a second anode layer of the first conductivity type having a lower doping concentration than the first anode layer and being separated from the anode electrode layer by the first anode layer, the second anode layer extending in the vertical direction from the first anode layer to a second depth that is larger than the first depth; a drift layer of a second conductivity type forming a pn-junction with the second anode layer, wherein the second conductivity type is different from the first conductivity type and wherein a horizontal cross-section through each diode cell along a horizontal plane perpendicular to the vertical direction at a third depth comprises, in each diode cell, a first area where the horizontal plane intersects the second anode layer and a second area where the horizontal plane intersects the drift layer, the first depth being smaller than the third depth and the third depth being smaller than the second depth and wherein, in a vertical cross-section perpendicular to the horizontal plane, a portion of the drift layer laterally separates the second anode layer in each diode cell into two separate regions laterally extending from the portion of the drift layer to an edge of the diode cell; a cathode layer of second conductivity type in direct contact with the cathode electrode layer, the cathode layer having a higher doping concentration than the drift layer; and a cathode-side segmentation layer in direct contact with the cathode electrode layer, wherein a material of the cathode-side segmentation layer is an insulating material or a lightly doped semiconductor of the first conductivity type having an integrated doping content along a direction perpendicular to a main side that is below 2·10¹³ cm⁻².
 32. The power diode according to claim 31, wherein the material of the cathode-side segmentation layer is an insulating material.
 33. The power diode according to claim 31, wherein the material of the cathode-side segmentation layer is a lightly doped semiconductor of the first conductivity type having a integrated doping content along a direction perpendicular to the main side that is below 2·10¹³ cm⁻².
 34. The power diode according to claim 31, wherein, in the vertical cross-section, the cathode layers of each pair of neighboring diode cells are laterally separated from each other by the cathode-side segmentation layers of the respective pair of neighboring diode cells.
 35. The power diode according to claim 31, wherein, in the vertical cross-section, a shortest lateral distance L_(d2) between the two separate regions of the second anode layer in each diode cell of each pair of neighboring diode cells is in a range from 0.3·L_(p2) to L_(p2), wherein L_(p2) is a shortest lateral distance between the cathode layers of the pair of neighboring diode cells in the vertical cross-section.
 36. The power diode according to claim 31, wherein, in the vertical cross-section, a lateral width L_(n2) of the cathode layer of each diode cell is in a range from 0.3·W_(n) to W_(n), wherein W_(n) is a vertical thickness of the diode cell.
 37. The power diode according to claim 31, wherein, in the vertical cross-section, a shortest lateral distance L_(p2) between the cathode layers of each pair of neighboring diode cells is in a range from 0.3·W_(n) to W_(n), wherein W_(n) is a vertical thickness of the diode cells.
 38. A method of forming a diode cell of a power diode that comprises an anode electrode layer, a cathode electrode layer and a plurality of diode cells arranged between the anode electrode layer and the cathode electrode layer, the method comprising: forming a first anode layer of a first conductivity type in direct contact with the anode electrode layer and extending in a vertical direction from the anode electrode layer to a first depth, the vertical direction being defined as a direction from the anode electrode layer to the cathode electrode layer; forming a second anode layer of the first conductivity type having a lower doping concentration than the first anode layer and being separated from the anode electrode layer by the first anode layer, the second anode layer extending in the vertical direction from the first anode layer to a second depth that is larger than the first depth; forming a drift layer of a second conductivity type forming a pn-junction with the second anode layer, wherein the second conductivity type is different from the first conductivity type and wherein a horizontal cross-section through the diode cell along a horizontal plane perpendicular to the vertical direction at a third depth comprises, in the diode cell, a first area where the horizontal plane intersects the second anode layer and a second area where the horizontal plane intersects the drift layer, the first depth being smaller than the third depth and the third depth being smaller than the second depth; forming a cathode layer of second conductivity type in direct contact with the cathode electrode layer, the cathode layer having a higher doping concentration than the drift layer; and forming a cathode-side segmentation layer in direct contact with the cathode electrode layer, wherein a material of the cathode-side segmentation layer is an insulating material or a lightly doped semiconductor of the first conductivity type having an integrated doping content along a direction perpendicular to a main side that is below 2·10¹³ cm⁻². 